This invention relates to a novel class of halofluorinated acrylates and more particularly to chlorofluorinated or bromofluorinated acrylates characterized by achlorofluorinated alkylene moiety with acrylate functions at both terminals. These chlorofluorinated or bromofluorinated acrylates may be photocured in the presence of a photoinitiator into transparent polymers useful as optical waveguiding materials.
The use of photocuring technology has grown rapidly within the last decade. Photocuring involves the radiation induced polymerization or crosslinking of monomers into a three dimensional network. The polymerization mechanism can be either radical or cationic. Radical initiated polymerization is the most common. Most commercial photocuring systems consist of multifunctional acrylate monomers and free radical photoinitiators. Photocuring has a number of advantages including: a 100% conversion to a solid composition, short cycle times and limited space and capital requirements.
Photocuring technology has recently been applied in planar waveguide applications. See, B. M. Monroe and W. K. Smothers, in Polymers for Lightwave and Integrated Optics, Technology and Applications, L. A. Hornak, ed., p. 145, Dekker, 1992. In its simplest application, a photocurable composition is applied to a substrate and irradiated with light in a predetermined pattern to produce (the light transmissive) or waveguide portion on the substrate. Photocurring permits one to record fine patterns ( less than 1 um) directly with light. The refractive index difference between the substrate and the light transmissive portion of the substrate can be controlled by either regulating the photocurable composition or the developing conditions.
Because of the dramatic growth in the telecommunications industry there is a need to develop photocurable compositions for optical waveguide and interconnect applications. In order to be useful in these applications, the photocurable composition must be highly transparent at the working wavelength and possess low intrinsic absorption and scattering loss. Unfortunately, in the near-infrared region, among which the 1300 and the 1550 nm wavelengths are preferred for optical communications, conventional photocurable materials possess neither the required transparency or low intrinsic absorption loss.
The absorption loss in the near-infrared stems from the high harmonics of bond vibrations of the Cxe2x80x94H bonds which comprise the basic molecules in conventional acrylate photopolymers. One way to shift the absorption bands to higher wavelength, is to replace most, if not all, of the hydrogen atoms in the conventional materials with heavier elements such as deuterium, fluorine, and chlorine. See, T. Kaino, in Polymers for Lightwave and Integrated Optics, Technology and Applications, L. A. Hornak, ed., p. 1, Dekker, 1992. The replacement of hydrogen atoms with fluorine atoms is the easiest of these methods. It is known in the art that optical loss at 1300 and 1550 nm can be significantly reduced by increasing the fluorine to hydrogen ratio in the polymer. It was recently reported that some perfluorinated polyimide polymers have very low absorption over the wavelengths used in optical communications. See, S. Ando, T. Matsuda, and S. Sasaki, Chemtech, 1994-12, p.20. Unfortunately, these materials are not photocurable.
U.S. Pat. No. 5,274,174 discloses a new class of photocurable compositions comprised of certain fluorinated monomers such as diacrylates with perfluoro or perfluoropolyether chains which possess low intrinsic absorption loss. It is, therefore, possible to make low loss optical interconnects from a photocurable system include these materials.
Fluorine substitution in the polymer structure, however, also induces some other less desirable changes in the polymer""s physical properties. One such change is the decrease in refractive index. For a highly fluorinated acrylate photopolymer, the refractive index decreases to the 1.32 region when the H/F mole ratio reaches 0.25. For optical interconnect applications, to avoid loss of light, it is important that the refractive index of the core of a planar waveguide approximate and preferably match that of the optical fiber (generally 1.45). Another problem with fluorine substitution in the polymer is the decrease of the surface energy of the resulting photopolymer film which results in its reduced adhesion to other materials like substrates.
It is also important to be abovel to precisely control and fine tune the refractive index of the photopolymer at the working wavelength in optical waveguide and interconnect applications. A desired index of refraction can be produced by mixing photocurable monomers with different refractive indices. Most photopolymers made from conventional photocurable monomers have refractive indices in the region of 1.45-1.55. Depending on the application, it is often desirable to lower a photopolymer""s refractive index. One way to do this is to mix low refractive index fluorinated monomers with conventional hydrocarbon-based monomers. Unfortunately this is difficult to accomplish because of the incompatibility or insolubility of the different monomer systems. Thus, there is a need for photocurable compositions which: (i) possess low optical loss in the near-infrared region, (ii) possess a refractive index approaching traditional optical fibers; and (iii) are compatible with both conventional hydrocarbon-based and highly fluorinated monomers.
The photocurable monomer of the invention is a di-, tri- or tetra-acrylate which contains a chlorofluorinated or bromofluorinated alkylene chain and has the formula: 
wherein 0 in an integer of from 2-4; X is H, F, CH3 or Cl and is preferably H, or Cl.
R=xe2x80x94CH2RFCH2xe2x80x94, 
and
RF=xe2x80x94(CF2CFX1)aCF2xe2x80x94, xe2x80x94(CF2CFX1)axe2x80x94(CFX2CF2)bxe2x80x94, xe2x80x94(CF2CFX1)axe2x80x94(CF2CFX2)bCF2xe2x80x94, and xe2x80x94(CF2CFX1)axe2x80x94(CH2CY1Y2)bxe2x80x94(CF2CFX1)dCF2xe2x80x94.
wherein X1 is Cl or Br; X2 is F, Cl or Br; Y, and Y2 are the same or different and are H, CH3, F, Cl or Br; and a, b, and c are the same or different and are integers of from 1-10 and preferably integers of from 1-7.
In the preferred embodiments, the above mentioned monomers contain chlorofluorinated or bromofluorinated alkylene chains which comprise chlorotrifluoroethylene or bromotrifluoroethylene repeating units and at least two terminal acrylate groups.
These monomers contain much less hydrogen than conventional photocurable monomers such that their inherent carbon-hydrogen bond absorption is greatly reduced. In addition, the introduction of chlorine or bromine atoms into the molecule offsets the effect of fluorine on the refractive index of the monomer producing a material with an index of refraction between about 1.40-1.48 As a result, the monomers of the invention are particularly useful in optical applications in the 1300-1550 nm wavelength region. The monomers are also compatible with both conventional hydrocarbon-based and highly fluorinated monomers. Because of this compatibility, it becomes possible to fine tune the refractive index and other physical properties of photocurable compositions containing these photocurable monomer.
In a second embodiment, the invention relates to a photocurable composition comprising at least one photocurable monomer of the invention and a photoinitiator.
In another embodiment, the invention relates to a process for producing an optical device containing a light transmissive region comprising:
a) applying a film of a photocurable composition comprising a photocurable monomer of the invention and a photoinitiator to a substrate; b) imagewise exposing said composition to sufficient actinic radiation to form exposed and unexposed areas on the substrate; and c) removing the unexposed portions of the composition.
In still another embodiment, the invention relates to an optical device comprising a light transmissive region wherein said light transmissive region comprises a photocurable composition of the invention.
In yet another embodiment, the invention relates to a process for the manufacture of an xcex1,xcfx89 Diol of the formula:
HOCH2xe2x80x94RFxe2x80x94CH2OH 
comprising reacting a xcex1,xcfx89-Diester of the formula, 
with aluminum hydride under conditions sufficient to produce said xcex1,xcfx89 Diol.
In still another embodiment the invention relates to a process for the production of an xcex1,xcfx89-diol of the formula HOCH2xe2x80x94RFxe2x80x94CH2OF which comprises reacting an xcex1,xcfx89-diester of the formula 
with aluminum hydride under conditions sufficient to produce said xcex1,xcfx89-diol;
wherein
R1 is a straight or branched chain alkyl group of from 1 to about 10 carbon atoms, and
RF=xe2x80x94(CF2CFX1)aCF2xe2x80x94,xe2x80x94(CF2CFX1)axe2x80x94(CFX2CF2)bxe2x80x94, xe2x80x94(CF2CFX1)axe2x80x94(CF2CFX2)bCF2xe2x80x94, or xe2x80x94(CF2CFX1)axe2x80x94(CH2CY1Y2)bxe2x80x94(CF2CFX1)cCF2xe2x80x94
wherein X1=Cl or Br; X2=F, Cl, or Br; Y, and Y2 are independently H, CH3, F, Cl, or Br; a, b, and c are independently integers from 1 to about 10.
All of the photocurable monomers of the invention may be made by using or adapting methods known in the art. Methods for the preparation of certain xcex1,xcfx89-diols with chlorofluorinated backbones are known. See, B. Boutevin, A. Rousseau, and D. Bosc, Jour. Polym. Sci., Part A, Polym. Chem., 30, 1279, 1993; B. Boutevin, A. Rousseau, and D. Bosc, Fiber and Integrated Optics, 13, 309, 1994; and B. Boutevin and Y. Pietrasanta, European Polym. Jour., 12, p.231, 1976.
The photocurable monomers of the invention may be prepared by following the general reaction scheme outlined below wherein RF, and X have the meanings set forth above and R1 is a straight or branched chain alkyl group of from 1-10 and preferably 1-3 carbon atoms. 
Preparation of the xcex1,xcfx89-Diester
Methods for the preparation of the xcex1,xcfx89-diester are known in the art. The diester may be prepared, for example by reducing with lithium aluminum hydride the appropriate telomer of a halotrifluoroethylene monomer. Examples of suitable halotrifluoroethylene monomers include: chlorotrifluoroethylene (or bromotrifluoroethylene), alone or mixture with other vinyl monomers such as bromotrifluoroethylene (or chlorotrifluoroethylene), tetrafluoroethylene, trifluoroethylene, vinylidene fluoride, vinyl fluoride, vinyl chloride, vinylidene chloride, ethylene, and propylene. Preparation of xcex1,xcfx89-diesters with a chlorofluorinated alkylene chain composed of chlorotrifluoroethylene monomer units is described in U.S. Pat. Nos. 2,806,865 and 2,806,866
The prior art processes discussed above produce mixtures of xcex1,xcfx89-diols because of the partial reduction of chlorine and bromine in the xcex1,xcfx89-diester to hydrogen. Applicants have unexpectedly found that a pure (not a mixture) xcex1,xcfx89-diol can be obtained by using an aluminum hydride reducing agent. Compare Example 1 and Examples 2-4 below.
Preparation of Triols and Tetraols
The xcex1,xcfx89-diols can be converted to triols and tetraols by methods known in the art. For example, a xcex1,xcfx89-triol can be obtained by: reacting the xcex1,xcfx89-diol with a metal hydroxide base or a metal alkoxide base to produce a metal salt of the xcex1,xcfx89-diol, reacting the metal salt with an allyl halide to produce an allyl ether of the xcex1,xcfx89-diol, and finally reacting the allyl ether with a peroxyacid to produce a xcex1,xcfx89-triol. See, Turri, S.; Scicchitano, M.; and Tonelli, C., Jour. Polymer Science: Part A: Polymer Chemistry, 1966, 34, p.3263.
Preparation of the Di-, Tri- and Tetra-acrylates
The di-, tri-, and tetra- acrylates can also be prepared by methods known in the art. For example, a triacrylate of the invention may be prepared by reacting the triol described above with an acryloyl halide in the presence of an organic base and an anhydrous aprotic solvent.
In addition to the photocurable monomer described above, other photocurable compounds which are known in the art may be incorporated into the photocurable compositions of the invention. These compounds include monomers, oligomers and polymers containing at least one terminal ethylenically unsaturated group and being capable of forming a high molecular weight polymer by free radical initiated, chain propagating addition polymerization.
Suitable monomers include, but are not limited to, ethers, esters and partial esters of acrylic and methacrylic acid; aromatic and aliphatic polyols containing from about 2 to about 30 carbon atoms, and cycloaliphatic polyols containing from about 5 to about 6 ring carbon atoms. Specific examples of compounds within these classes are: ethylene glycol diacrylate and dimethacrylate, diethylene glycol diacrylate and dimethacrylate, triethylene glycol diacrylate and dimethacrylate, hexane diacrylate and dimethacrylate, trimethylolpropane triacrylate and trimethacrylate, dipentaerythritol pentaacrylate, pentaerythritol triacrylate and trimethacrylate, alkoxylated bisphenol-A diacrylates and dimethacrylates (e g, ethoxylated bisphenol-A di-acrylate and dimethacrylate), propoxylated bisphenol-A diacrylates and dimethacrylates, ethoxylated hexafluorobisphenol-A diacrylates and dimethacrylates and mixtures of the above compounds. Preferred monomers include multifunctional aryl acrylates and methacrylates. More preferred aryl acrylate monomers include di-, tri- and tetra-acrylates and methacrylates based on the bisphenol-A structure. Most preferred aryl acrylate monomers are alkoxylated bisphenol-A diacrylates and dimethacrylates such as ethoxylated bisphenol-A di-acrylates and dimethacrylates, and ethoxylated hexafluorobisphenol-A diacrylates and dimethacrylates.
Suitable oligomers include, but are not limited to, epoxy acrylate oligomers, aliphatic and aromatic urethane acrylate oligomers, polyester acrylate oligomers, and acrylated acrylic oligomers. Epoxy acrylate oligomers (such as Ebercryl 600 by Radcure) are preferred.
Suitable polymers include, but are not limited to, acrylated polyvinyl alcohol, polyester acrylates and methacrylates, acrylated and methacrylated styrene-maleic acid copolymers. Acrylated styrene-maleic acid copolymers such as Sarbox SR-454 sold by Sartomer are preferred.
The photocurable component is comprised of photocurable monomer A, and optionally the other photocurable compounds described above. The photocurable component is present in an amount sufficient to photocure and provide image differentiation upon exposure to sufficient actinic radiation. The amount of the photocurable component in the photocurable composition may vary widely. Typically the photocurable component is present in an amount of from about 35 to about 99% by weight of the overall composition. In a preferred embodiment, the photocurable component is present in an amount of from about 80 to about 99% by weight and more preferably from about 95 to about 99% by weight in the overall composition. The weight ratio of monomer A to the other photocurable compounds may vary from about 1:9 to about 9:1. Preferably the weight ratio ranges from about 1:1 to about 9:1.
The photocurable composition further comprises at least one photoinitiator which photolytically generates activated species capable of inducing polymerization. Any photoinitiator known to be useful in the polymerization of acrylates or methacrylates may be used in the photocurable compositions of the invention. Suitable photoinitiators include aromatic ketone derivatives such as benzophenone, acrylated benzophenone, phenanthraquinone, 2,3-dichloronaphthoquinone, benzyl dimethyl ketal and other aromatic ketones (e.g. benzoin), benzoin ethers such as benzoin methyl ether, benzoin ethyl ether, benzoin isobutyl ether and benzoin phenyl ether. Preferred photoinitiators include 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184), benzoin, benzoin ethyl ether, benzoin isopropyl ether, benzophenone, benzodimethyl ketal (Irgacure 651). xcex1,xcex1-diethyloxy acetophenone, xcex1,xcex1-dimethyloxy-xcex1-hydroxyacetophenone (Darocur 1173), 1-[4-(2-hydroxyethoxy)phenyl]-2-hydroxy-2-methyl-propan-1-one (Darocur 2959), 2-methyl-1-[4-methylthio)phenyl]-2-morpholino-propan-1-one (Irgacure 907), 2-benzyl-2-dimethylamino-1-(4-morpholinophenyl)-butan-1-one (Irgacure 369), poly{1-[4-(1-methylvinyl)phenyl]-2-hydroxy-2-methyl-propan-1-one } (Esacure KIP). [4-(4-methylphenylthio)-phenyl]- phenylmethanone (Quantacure BMS), and dicampherquinone. The most preferred photoinutiators are those which tend not to yellow upon irradiation. Such photoinitiators include benzodimethyl ketal (Irgacure 651), xcex1,xcex1-dimethyloxy-xcex1-hydroxy acetophenone (Darocur 1173), 1-hydroxy-cyclohexyl-phenyl ketone (Irgacure 184), and 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-propan-1-one (Darocur 2959).
The photoinitiator is present in an amount sufficient to effect photopolymerization of the photocurable compound upon exposure to sufficient actinic radiation. The photoinitiator may comprise from about 0.01 to about 10% by weight preferably from about 0.1 to about 6% by weight and most preferably from about 0.5 to about 4% by weight based upon the total weight of the photocurable composition.
Various optional additives may also be added to the photocurable compositions of the invention depending upon the application in which such they are to be used. Examples of these optional additives include antioxidants, photostabilizers, volume expanders, fillers (e.g., silica and glass spheres), dyes, free radical scavengers, contrast enhancers and UV absorbers. Antioxidants include such compounds as phenols and particularly hindered phenols including Irganox 1010 from Ciba-Geigy; sulfides; organoboron compounds; organophosphorous compounds; and N, Nxe2x80x2-hexamethylenebis(3,5-di-ter(-butyl-4-hydroxyhydrocinnamamide) available from Ciba-Geigy under the tradename Irganox 1098. Photostabilizers and more particularly hindered amine light stabilizers include, but are not limited to, poly[(6-hexamethylene [2,2,6,6-tetramethyl-4-piperidyl)imino)] available from Cytec Industries under the tradename Cyasorb UV3346. Volume expanding compounds include such materials as the spiral monomers known as Bailey""s monomer. Suitable dyes include methylene green, methylene blue, and the like. Suitable free radical scavengers include oxygen, hindered amine light stabilizers, hindered phenols, and 2,2,6,6--tetramethyl-1-piperidinyloxy free radical (TEMPO). Suitable contrast enhancers include other free radical scavengers. UV absorbers include benzotriazoles and hydroxybenzophenone. These additives may be included in quantities, based upon the total weight of the composition of from about 0 to about 6%, and preferably from about 0.1% to about 1%. Preferably all components of the photocurable composition are in admixture with one another and more preferably in a substantially uniform admixture.
The photocurable compositions of this invention can be used in the formation of the light transmissive element of an optical device. Illustrative of such devices are planar optical slab waveguides, channel optical waveguides, ribbed waveguides, optical couplers, and splitters. The photocurable composition of this invention can also be used in the formation of negative working photoresists and other lithographic elements such as printing plates. In a preferred embodiment of the invention, the photocurable composition is used for producing a waveguide comprising a substrate containing a light transmissive element. Such waveguides are formed by applying a film of the photocurable composition of the invention to the surface of a suitable substrate. The film may, be formed by any method known in the art, such as spin coating, dip coating, slot coating, roller coating, doctor blading, and evaporation.
The substrate may be any material on which it is desired to establish a waveguide including semiconductor materials such as silicon, silicon oxide and gallium arsenide. In the event that the light transmissive region on the substrate is to be made from a photocurable material which has an index of refraction which is lower than that of the substrate, an intermediate buffer layer possessing an index of refraction which is lower than the substrate must be applied to the substrate before the photocurable composition is added. Otherwise, the light loss in the waveguide will be unacceptable. Suitable buffers are made from semiconductor oxides, lower refractive index polymers or spin-on silicon dioxide glass materials.
Once a film of the photocurable composition is applied to the substrate, actinic radiation is directed onto the film in order to delineate the light transmissive region. That is, the position and dimensions of the light transmissive device are determined by the pattern of the actinic radiation upon the surface of the film on the substrate. The photopolymers of the invention are conventionally prepared by exposing the photocurable composition to sufficient actinic radiation. For purposes of this invention, xe2x80x9csufficient actinic radiationxe2x80x9d means light energy of the required wavelength, intensity and duration to produce the desired degree of polymerize action in the photocurable composition. Suitable sources of actinic radiation include light in the visible, ultraviolet or infrared regions of the spectrum, as well as electron beam, ion or neutron beam or X-ray radiation. Actinic radiation may be in the form of incoherent light or coherent light such as light from a laser.
Sources of actinic light, exposure procedures, times, wavelengths and intensities may vary widely depending on the desired degree of polymerization, the index of refraction of the photopolymer and other factors known to those of ordinary skill in the art. The selection and optimization of these factors are well known to those skilled in the art.
It is preferred that the photochemical excitation be carried out with relatively short wavelengths (or high energy) radiation so that exposure to radiation normally encountered before processing (e.g., room lights) will not prematurely polymerize the polymerizable material. The energy necessary to polymerize the photocurable compositions of the invention generally ranges from about 5 mW/cm2 to about 100 mW/cm2 with typical exposure times ranging from about 0.1 second to about 5 minutes.
After the photocurable composition has been polymerized to form a predetermined pattern on the surface of the substrate, the pattern is then developed to remove the nonimage areas. Any conventional development method can be used such as flushing the unirradiated composition with a solvent. Suitable solvents include polar solvents, such as alcohols and ketones. The most preferred solvents are acetone, methanol, tetrahydrofuran and ethyl acetate.
The following non-limiting examples serve to illustrate the invention.